Not sure what you mean by “delete”? Do you mean from your controller, or from your “account” on www.Sonos.com

The first should age out as the lease is renewed for that IP, I suspect. The second requires the new owner of the device to register it, which is one of the reasons you should always  factory reset a device when getting rid of it, it will force the new user to register it to their own account, which removes it from yours. 

The device is dead. So total delete

It will be removed from your system after not being connected for soem time. Just be patient - if you need the room name the dead speaker is occupying know that room names can be changed.

Ok. The main reason is that i need the device name and I can't change it when it's not online.

It will not be deleted from your ‘purchased’ device list on www.Sonos.com under your account there, but you may be able to ‘rush’ it being deleted from your controller by rebooting your router. 

Ok. Thank you 

Just follow the ‘not connected’ link in your Sonos App and it will take you to a screen where you can choose to hide the product (you need to sign in to hide it).

Thank you I know. But I want to delete it. It is on the junkyard and I want to reuse the name :-)

Maybe hiding it will also free up the name (I haven’t used this feature myself).

If the device is hidden - it never appears again in the Sonos App until it’s powered back on, on the same local subnet. Another device can be given the same ‘room’ name. It shouldn’t cause an issue, but perhaps check it’s removed from other areas, particularly if using smart-home hubs/voice assistants etc; but I would personally always check those things anyway.